The invention relates to a valve seat for a metering valve of an exhaust-gas after-treatment system.
Diesel internal-combustion engines and lean-burn spark-ignition internal-combustion engines, in which fuel is injected directly into the combustion chamber in the cylinder, have a tendency toward high NOx emissions because of the high excess of oxygen with which they are operated. The selective catalytic reduction (SCR) process is known to reduce the NOx content in the exhaust gas from such internal-combustion engines. In this process, a reducing agent is introduced into the exhaust-gas stream at a point upstream of a reduction catalytic converter, with the aid of an electrically driven metering device, and converts the nitrogen oxides contained in the exhaust gas into harmless nitrogen N2 and water H2O in the presence of oxygen at the reduction catalytic converter. A process of this type is described in the publication xe2x80x9cEin geregeltes Abgasnachbehandlungssystem zur Erfxc3xcllung zukxc3xcnftiger Emissionsgrenzwerte bei Pkw-Dieselmotorenxe2x80x9d [A controlled exhaust-gas after-treatment system for meeting future emission limit values in passenger-automobile diesel engines] by Schxc3x6ppe et al., 17th International Vienna Motor Symposium, 1996, Vol. 1. The reducing agent used can be ammonia NH3, but for reasons of its ability to be handled, an aqueous urea solution is normally used. However, it is also possible to use fuel or derivatives as reducing agent.
In such exhaust-gas after-treatment systems, a control device of the internal-combustion engine, or a separate control device, often referred to as a metering control device or DENOx control device, continuously calculates the desired quantity of reducing agent to be metered on the basis of operating parameters of the internal-combustion engine, such as for example the quantity of fuel introduced into the combustion chamber, the operating temperature and the rotational speed, as well as the temperature of the reduction catalytic converter.
The metering device used in this case is preferably a conventional injection valve, as is used, for example, in a low-pressure gasoline injection system. In accordance with the changed intended use of such an injection valve (metering valve), namely of injecting reducing agent, for example aqueous urea solution, at least those components of the valve which are directly exposed to the urea solution are produced from urea-resistant materials (stainless steel, coated metal, plastic), and the geometry of the outlet opening of the valve is adapted to the quantities to be ejected, which are smaller than when metering fuel.
Since, in SCR systems of this type, the reducing agent is introduced directly into the exhaust gas, the metering valve is attached to the exhaust pipe using a suitable seat device or adapter in such a manner that the jet of liquid penetrates into the hot exhaust-gas stream and is evaporated. The liquid should be prevented from wetting the wall of the adapter or of the exhaust pipe, since in certain temperature ranges this may lead to deposits.
Under unfavorable conditions, the exhaust-gas temperatures at the location of the metering valve may reach more than 500xc2x0 C. This may have adverse effects on the reducing agent which is to be introduced and even on the metering valve itself.
The use of conventional low-pressure gasoline injection valves is generally permissible only up to a specific temperature, typically about 130xc2x0 C. The reason for this lies in the coil construction, in particular the insulation of the coils for the electromagnetic drive and the thermal resistances of the materials used, for example in the seals. However, if the injection valve or at least parts thereof are heated above this maximum permissible temperature for a relatively long time, functional disruption or even the complete failure of the injection valve may occur.
When using aqueous urea solution as reducing agent, this substance must not be heated above defined temperature limits (typical upper limit approx. 70xc2x0 C.) for a prolonged period, since above this critical temperature the urea begins to be converted into other chemical compounds, and effective exhaust-gas after-treatment is no longer ensured.
The adapter which receives the metering valve must therefore allow a high temperature gradient between exhaust pipe and metering valve, so that despite the valve being fitted to the hot exhaust pipe, cooling which is as efficient as possible can be achieved.
DE 44 36 397 A1 discloses that in order to introduce a reducing agent into the exhaust gas fed to the reduction catalytic converter, an electrically controlled metering valve is provided, which is combined with a control valve in a common housing. The control valve is used for the controlled introduction of supplied compressed air, in which a quantity of reducing agent stored via the metering valve is processed and added intermittently to the exhaust gas. The control valve and the metering valve are arranged in a common supporting body, around which cooling water from the cooling-water circuit of the internal-combustion engine flows. Although, by means of such a configuration, the maximum temperature at the end of the injection valve seat can be limited to the cooling-water temperature (max. 90xc2x0-100xc2x0 C.) and the temperature stress on the urea circuit can be relieved, a cooling jacket, lines and hose connections are additionally needed.
DE 38 24 954 A1 describes a cleaning system for the exhaust gases from an internal-combustion engine, with a catalytic converter arranged in a housing, an exhaust-gas line between the internal-combustion engine and the catalytic converter, in which a heat pipe, in its evaporation zone, extracts heat from the exhaust gases fed to the catalytic converter and releases this heat via a condensation zone which lies in an environment which is at a low temperature. In this case, the evaporation zone of the uncontrolled heat pipe is arranged in the exhaust-gas line upstream of the catalytic converter housing.
The present invention is based on the object of providing a seat device for a metering valve of an exhaust-gas after-treatment system for an internal-combustion engine which is operated with an excess of air. The seat device is of simple structure and minimizes the thermal loading on the metering valve and on the reducing agent. The use of an uncontrolled heat pipe in the valve seat device for transferring the waste heat from the exhaust gas acting on the metering valve to a condensation zone which lies in an environment which is at a lower temperature makes it possible to cool the metering valve and to prevent a chemical change in the reducing agent as a result of the high temperatures.
A cavity in the valve seat device forms a cooling ring which surrounds the metering valve tip. The ring is filled with a liquid and is in flow communication with a heat sink, e.g. a cooling body, which, in order to increase the surface area and therefore the cooling action, is provided with ribs. An arrangement of this type has the advantage that there is no need for a separate coolant circuit or components for active cooling (blower, coolant pump), including electrical or hydraulic elements. It has no moving parts whatsoever; it is completely maintenance-free; and on account of its simplicity can be used at low cost. The heat sink can be integrated into the vehicle without problems since its shape can substantially be matched as desired to installation conditions, and only a relatively small amount of heat has to be dissipated per unit time.
Particularly efficient cooling of the metering valve results if the heat sink is designed as a cooling body which is arranged at a location in the vehicle which is exposed to the flow either of the slipstream or of the air stream of the cooling fan. To increase the cooling capacity, it is also possible to provide a plurality of heat pipes on the valve seat device, and these heat pipes are either all connected to one common cooling body or are each assigned a dedicated cooling body.
Overall, the use of the heat pipe principle results in the simple and inexpensive cooling of the metering valve. The reducing agent is no longer increased to above the critical temperature, and there is therefore no thermal decomposition of the reducing agent.